1. Field of the Invention
This invention relates to a laser device. More specifically, the present invention relates to a quantum cascade laser device having repetitively arranged active regions. The present invention particularly relates to a quantum cascade laser device adapted to operate in a frequency band within a frequency range extending from the millimeter wave band to the terahertz band (30 GHz to 30 THz).
2. Description of the Related Art
New type semiconductor lasers based on intersubband transitions of carriers in the conduction band or in the valence band, hence within a single energy band, are known and referred to as quantum cascade lasers. Since the oscillation wavelength of a quantum cascade laser depends on the energy gap of two subbands involved in the optical transition, it can be selected from a broad spectral range (extending from the mid-infrared region to the terahertz band). Japanese Patent Application Laid-Open No. H08-279647 disclosed for the first time that such a laser can be realized by way of an arrangement of selecting an oscillation wavelength of 4.2 μm in the mid-infrared region.
Recently, longer wavelength lasers have been developed to exploit a long wavelength region relative to the mid-infrared region for the oscillation wavelength in order to meet the demand for electromagnetic wave resources of the terahertz band that is believed to be useful for biosensing applications. Nature, Vol. 417, 156 (2002), describes a laser oscillation at 67 μm (4.5 THz) in the terahertz band. Appl. Phys. Lett., Vol. 83, 2124 (2003), describes a laser oscillation at a longer wavelength of about 100 μm (3 THz), involving a surface plasmon waveguide, which includes a negative permittivity medium, the real part of permittivity thereof being negative for the oscillation wavelength.
The configuration of a quantum cascade laser will be summarily described below by referring to FIGS. 4A and 4B of the accompanying drawings, which also illustrate the band profiles employed in the example as will be described hereinafter.
FIG. 4A illustrates part of the conduction band structure when a designed electric field is applied to a quantum cascade laser. Referring to FIG. 4A, active region 440 is formed by barriers 441, 443 and 445 and quantum wells 442, 444 and 446. With this arrangement, subbands 411, 412 and 413 are formed in the active region 440. Relaxation region 450 is formed by barriers 451, 453, 455 and 457 and quantum wells 452, 454, 456 and 458. With this arrangement, a miniband 421 bundling a plurality of subbands is formed. Quantum cascade lasers are characterized in that active regions and relaxation regions are repetitively and alternately arranged. In FIG. 4A, active region 460 is the active region that appears next in the repetitive arrangement. When a designed electric field is applied to such a quantum cascade laser, an electric current occurs in a manner as described below.
Electrons make an optical transition 401 from the subband 411 to the subband 412 in the active region 440 to emit light of a wavelength that corresponds to the energy gap between the subband 411 and the subband 412. Subsequently, the electrons in the subband 412 of the active region 440 pass through the subband 413 due to optical phonon scattering 402 so as to be extracted into a relaxation region 450, securing the population inversion between the subband 411 and the subband 412. Then, the electrons pass through the miniband 421 in the relaxation region 450 and are injected into the next active region 460 to repeat the same optical transition as in the active region 440. The relaxation region 450 is referred to as “injector” because the relaxation region 450 injects carriers into the next repeated active region. One or more than one of the quantum wells in a relaxation region are slightly doped with carriers.